Congressional Republicans have long boasted that once they claim the reins of power, they will act quickly and decisively to roll back what they view as the most onerous piece of President Obama’s domestic agenda: the Affordable Care Act.

But their actions starting Tuesday to end Obamacare will be far less sweeping, at least initially, than a full-blown repeal of the law.

Democratic opposition and complex Senate rules mean that core pieces of the 2010 health-care overhaul are likely to remain, including the legal framework for the individual mandate and pieces of the state exchanges the law created. Furthermore, President-elect Donald Trump has vowed to preserve other key aspects, such as a ban on insurers denying coverage because of preexisting conditions and a requirement that insurers cover children under 26 on their parents’ plans.

And while Republicans are determined to rapidly repeal as much of Obamacare as they can, they have not settled on a replacement plan or on when that plan should take effect. …